The invention relates to a head assembly for a nuclear reactor pressure vessel and more particularly to a simplified integrated head assembly including a missile shield disposed above control rod drive mechanisms operatively extending through the closure head of the pressure vessel.
Pressure vessels containing fuel assemblies in commercial pressurized water nuclear reactor facilities have control rods which are operated by control rod drive mechanism assemblies (CRDMs). The CRDMs are mechanically supported on a removable closure head bolted to the pressure vessel and laterally supported by a seismic support platform and vertically restrained by a missile shield. Missile shields are generally relatively large heavy concrete or metal structures designed to absorb kinetic energy from dislocated CRDMs or other objects originally attached to the reactor pressure vessel. In addition, the closure head must also mechanically support a complex ventilation system located above the closure head for providing a substantial, continuous flow of ambient containment air through the CRDM coil region. See, in this regard, FIG. 1 of U.S. Pat. No. 4,678,623 which shows a head arrangement found in many commercial facilities. Briefly, FIG. 1 shows a design wherein the surrounding ambient air below the seismic support platform 28 is drawn across the unbaffled upper portion of the CRDMs, downwardly along the baffled electromagnetic coils of the CRDMs, into a lower plenum 20, upwardly through ducts 22 into an upper plenum 24 and then exhausted by fans 26 into the surrounding atmosphere above the fans. By exhausting the air upwardly in this manner, the hot exhaust air from the CRDMs can be blown into the general containment atmosphere so that the walls of the refueling canal near the head of the reactor vessel are not be substantially heated. Concrete walls should not be exposed to temperatures of about 150.degree. F. and are preferably not exposed to temperatures of more than 120.degree. F. In addition to the ventilation system, and as is shown in FIG. 1 of U.S. Pat. No. 4,828,789, a closure head may also support a shield surrounding the CRDM assemblies for protecting maintenance workers from radioactive CRDM assemblies.
During refueling operations, the closure head, CRDM assemblies and their supporting subsystems, missile shield and other devices located over the closure head must be disassembled, lifted and removed so that the closure head can be removed and the spent fuel assemblies in the core of the pressure vessel below can be rearranged or replaced with fresh fuel assemblies. To reduce the time required to remove a closure head in order to refuel a nuclear reactor, an integrated head assembly was developed in the 1980s as a backfit for the design discussed above. As is shown in FIG. 2 of U.S. Pat. No. 4,678,623, the integrated head assembly replaced the ducts 22 extending from the lower plenum to the upper plenum with a duct arrangement 136, 138 and 140 which partially encircled the CRDMs.
In this type of arrangement, the surrounding ambient air below the seismic support 128 was drawn along the exposed upper portion of the CRDMs, downwardly past the baffled electromagnetic coils and into the lower plenum 120, upwardly through the ducts 136-140 into the upper plena 162, and then exhausted into the atmosphere by fans 126. Advantageously, this and other equipment was supported by the closure head during power operations and could be lifted as a unit from the closure head during refueling operations. See, also, U.S. Pat. No. 4,830,814 and UK Patent Application No. 2,100,496 which show another integrated head assembly design. While the integrated head assembly design introduced in the 1980s successfully provided its intended advantages quite well, it has proven to be difficult to backfit all of the associated assemblies and subassemblies in a radioactive, operating nuclear facility.
A natural convection ventilation design which could be more readily backfitted in an operating facility such as the reactor shown in FIG. 1 of U.S. Pat. No. 4,678,623 previously was proposed for cooling the CRDM coils without the need for any ventilation fans. It was determined in developing a natural convection ventilation design that the closure head arrangements employing the above described forced circulation type of ventilation systems utilized most of the fan power to draw the cooling air through the air ducts and utilized relatively little power to circulate the cooling air through the CRDM electromagnetic coil region.
This natural circulation design included a redesigned, taller cooling shroud to increase the natural draft and other features for reducing the air flow resistance through the head region. However, a finite element analysis of the natural circulation design indicated that natural convection could achieve only about one-fourth of the 48,000 cubic feet per minute (CFM) air flow required by the design. Calculations showed that such an arrangement could result in a peak coil surface temperatures of up to about 380.degree. F. based upon a continuous stepping heat load estimate of 12 kw/CRDM. Although such conditions may be considered acceptable based on the peak specified allowable temperature of 392.degree. F. at the coils, this design provides an assured temperature margin of only 32.degree. F., which might require more frequent replacement of the coils after many years of operation at the increased temperatures.
Thus, the nuclear industry has not developed an entirely satisfactory integrated head assembly design which will substantially cool the CRDM assemblies, reduce refueling time and radiation exposure.